A pulse oximeter is a physiological instrument that provides noninvasive measurements of arterial oxygen saturation along with pulse rate. To make these measurements, a pulse oximeter performs a spectral analysis of the pulsatile component of arterial blood so as to determine the relative concentration of oxygenated hemoglobin, the major oxygen carrying constituent of blood. Pulse oximeters provide early detection of decreases in the arterial oxygen supply, reducing the risk of accidental death and injury. As a result, these instruments have gained rapid acceptance in a wide variety of medical applications, including surgical wards, intensive care units, general wards and home care.
FIG. 1 illustrates a pulse oximetry system 100 having a sensor 110 and a monitor 120. The monitor 120 may be a multi-parameter patient monitor or a standalone, portable or handheld pulse oximeter. Further, the monitor 120 may be a pulse oximeter 200, such as an OEM printed circuit board (PCB), integrated with a host instrument including a host processor 122, as shown. The sensor 110 attaches to a patient and receives drive current from, and provides physiological signals to, the pulse oximeter 200. An external computer (PC) 130 may be used to communicate with the pulse oximeter 200 via the host processor 122. In particular, the PC 130 can be used to download firmware updates to the pulse oximeter 200 via the host processor 122, as described below.
FIG. 2 illustrates further detail of the pulse oximetry system 100. The sensor 110 has emitters 112 and a detector 114. The emitters 112 typically consist of a red light emitting diode (LED) and an infrared LED that project light through blood vessels and capillaries underneath a tissue site, such as a fingernail bed. The detector 114 is typically a photodiode positioned opposite the LEDs so as to detect the emitted light as it emerges from the tissue site. A pulse oximetry sensor is described in U.S. Pat. No. 6,088,607 entitled “Low Noise Optical Probe,” which is assigned to Masimo Corporation, Irvine, Calif. and incorporated by reference herein.
As shown in FIG. 2, the pulse oximeter 200 has a preamp 220, signal conditioning 230, an analog-to-digital converter (ADC) 240, a digital signal processor (DSP) 250, a drive controller 260 and LED drivers 270. The drivers 270 alternately activate the emitters 112 as determined by the controller 260. The preamp 220, signal conditioning 230 and ADC 240 provide an analog front-end that amplifies, filters and digitizes the current generated by the detector 114, which is proportional to the intensity of the light detected after tissue absorption in response to the emitters 112. The DSP 250 inputs the digitized, conditioned detector signal 242 and determines oxygen saturation, which is based upon the differential absorption by arterial blood of the two wavelengths projected by the emitters 112. Specifically, a ratio of detected red and infrared intensities is calculated by the DSP 250, and arterial oxygen saturation values are empirically determined based upon the ratio obtained. Oxygen saturation and calculated pulse rate values are communicated to the host processor 122 for display by the monitor 120 (FIG. 1). A pulse oximeter is described in U.S. Pat. No. 6,236,872 entitled “Signal Processing Apparatus,” which is assigned to Masimo Corporation, Irvine, CA and incorporated by reference herein.
Further shown in FIG. 2, the pulse oximeter 200 has a sensor port 210 and a communications port 280. The sensor port 210 includes a connector and associated input and output signals and provides an analog connection to the sensor 110. In particular, the sensor port 210 transmits a drive signal 212 to the LED emitters 112 from the LED drivers 270 and receives a physiological signal 214 from the photodiode detector 114 in response to the LED emitters 112, as described above. The communication port 280 also includes a connector and associated input and output signals and provides a bi-directional communication path 282 between the pulse oximeter 200 and the host processor 122. The communication path 282 allows the DSP 250 to transmit oxygen saturation and pulse rate values to the monitor 120 (FIG. 1), as described above. The communication path 282 also allows the DSP firmware to be updated, as described below.
Additionally shown in FIG. 2, the pulse oximeter 200 has a micro-controller 290 and a flash memory 255. The flash memory 255 holds the stored program or firmware that executes on the DSP 250 to compute oxygen saturation and pulse rate. The micro-controller 290 controls data transfers between the DSP 250 and the host processor 122. In particular, to update the DSP firmware, the firmware is uploaded into the PC 130 (FIG. 1), which downloads the firmware to the host processor 122. In turn, the host processor 122 downloads the firmware to the micro-controller 290, which downloads it to the DSP 250. Finally, the DSP 250 writes the firmware to the flash memory 255.